I used PVC pipe to construct my MW AM loop. Nearby metal can influence the pattern. I am mainly concerned about being able to produce sharp nulls to reduce signals from interfering stations on the same frequency, so wish to keep the pattern "clean".
I know that MW broadcasters in this country
must pay special attention to nearby metal towers (such a cell phone and TV towers) even to the extent of installing tuned "skirts" on said towers to reduce their effects on the MW station transmit pattern.
jr